Increased-valence structures and hypervalent molecules

Abstract

rnHypervalent molecules may involve the use of increased-valence structures to provide valence bond descriptions of their electronic structure. For electron-rich molecules with four electrons distributed among three overlapping (nuclear-centered) atomic orbitals, the increased-valence structures are Y-A * B and ? -A--B. Each structure involves a fractional electron-pair bond and a one-electron bond. It is deduced that the Armstrong-Perkins-Stewart valence of the A atom is able to exceed unity in each of these structures when the three bonding electrons occupy nonorthogonal localized molecular orbitals. It is also shown that increased valence for the A atom does not occur when the four electrons occupy localized molecular orbitals to give the valence-bond structure Y-A-B with three overlapping atomic orbitals, and the same number of orbital variational parameters as occurs in the wave functions for either of the increasedvalence structures. The results of ab initio valence bond calculations with minimal basis sets are reported for HT1, CH;, HF,, F,, CIF,, and FF,, and the resulting wave functions for resonance between six canonical Lewis structures are related to those for resonance between the two increased-valence structures. The use of the latter structures to indicate how electronic reorganization proceeds via one-electron delocalizations for S,Z reactions is redescribed, and an elementary argument is presented to deduce that this class of reactions cannot involve the delocalization of a pair of electrons in concert from the nucleophile. Increased-valence wave functions are used to deduce an expression for the avoided crossing for the transition state of the identity S , Z reaction.

Revisiting a Historical Concept by Using Quantum Crystallography: Are Phosphate, Sulfate and Perchlorate Anions Hypervalent?

“…Consequently, an atom may follow the modified octet rule, although the original version of the rule is violated . Furthermore, there is the theory of recoupled bonding and increased‐valence structures for hypervalency, as well as a classification by Green and Parkin …”

Symmetry Analysis in Mechanistic Studies of Nucleophilic Substitution and β-Elimination Reactions

“…Harcourt [7], on the other hand, argued that an S N 2 reaction is effected via consecutive one-electron transfers ( Figure 1b). Accordingly, the transition state contains a delocalized one-electron intermolecular bond.…”

Tutorial review: Bohr circular orbit diagrams for some fluorine-containing molecules

“…It is to be noted that in orbital theory, the singlet diradical structure of type 9, as well as the ionic structures of types 7 and 8, is also considered to make a substantial contribution to the ground-state resonance scheme for each of NF 5 and PF 5 [7 (b),13,14-16], but because of the above ionisation potential differences, the singlet diradical structure should be more important for NF 5 than it is for PF 5 . The results of STO-6G ab initio VB calculations [14] show that analogous theory is also appropriate to rationalise the existence of AF 3 when A = Cl and the non-existence of AF 3 when A = F. When discussing qualitatively the electronic structure of NF 5 using VB methods, consideration has not been given to the contribution of the singlet diradical structure to the groundstate resonance scheme [17].…”

“…These types of VB structures, such as 5 and 6, are used in VSEPR theory [3,10,21]. It is stressed that when A is an atom of a main-group element, these types of VB structures, which involve an expansion of the A-atom valence-shell, are not necessarily the primary VB structures for the ground-states of the species considered [13][14][15][16].…”